A wide variety of products are now packaged for dispensing as a finely divided aerosol spray. For example, such products include hair sprays, deodorants, food products, etc. While many products have been successfully dispensed as aerosols, a wide variety of products are, however, not amenable to being dispensed in this manner.
By way of background, in dispensing a product as an aerosol, the product which is usually in the form of a liquid, is placed within a pressurizable container and a propellant is then added to the product. Then, during dispensing of the product as an aerosol, the mixture of product and propellant is discharged from the container through a spray nozzle with the product being discharged in the form of a fine spray.
In general, there are two main types of propellants which may be used for aerosol systems, these being hydrocarbon propellants and halogenated hydrocarbon propellants. Both types of propellants have their drawbacks and, thus, in practice the particular propellant which is employed for a particular application is dictated by the nature of the product which is being dispensed.
To generate the pressure required to dispense a product in the form of an aerosol, the propellant must, in general, be present in a sufficient amount in admixture with the liquid product to carry the product through the spray head and to generate an internal pressure within the product container to accomplish this, e.g., generally about 30 psig or higher. The particle size of the product, after its discharge through a spray head, is a function of the viscosity of the product and also the pressure generated by the aerosol propellant. In general, as the viscosity of the product is increased, the size of the product particles discharged through the spray head is increased. Conversely, as the propellant pressure is increased, the size of the product particles is reduced.
Based on the above considerations, it might appear that any liquid product, no matter how viscous, could be packaged as an aerosol merely by adding sufficient propellant to generate a driving force that would be adequate to break up the product into a fine spray on discharge of the product through a spray head. In fact, however, the formulation of a successful aerosol system is complicated by a host of other factors which make it impossible to arrive at a satisfactory system merely by increasing the internal propellant pressure to whatever level may be dictated by the viscosity of the product.
As a first consideration, many propellants are incompatible with specific products at the relatively high propellant levels required to generate the necessary propellant pressure for discharge of the product as a fine spray. For example, many aerosol systems require a propellant-to-product volume ratio of about 50:50 to generate a sufficient propellant pressure to eject the product as a fine spray. In fact, some products, such as a frying pan spray, may require a propellant-to-product volume ratio of as high as about 95:5 to break up the viscous vegetable oil product into a fine spray.
At high propellant levels, the internal pressure generated by the propellant within an aerosol container may pose substantial safety problems unless the container is specially constructed to withstand high internal pressures. The vapor pressure of the propellant will vary with the ambient temperature which may be as high as 110.degree. to 120.degree. F. in some areas where the product is used. Even though an aerosol system is designed for an ambient temperature of about 70.degree. F., the propellant container must, therefore, be sufficiently strong to withstand the higher pressures which are developed by the propellant at higher ambient temperatures. The cost of the aerosol product will be determined, to a large extent, by the cost of the aerosol container. Thus, if the pressure demands of an aerosol system require the use of an expensive high pressure aerosol container, the aerosol product may be economically unfeasible.
Further, as the propellant-to-product volume ratio is increased, the amount of product is reduced. This may make the aerosol product unattractive to the consumer who must then purchase a relatively large quantity of propellant to obtain a relatively small quantity of product. This situation is illustrated by aerosol frying pan sprays where the consumer may have to purchase 95 percent by volume of propellant merely to obtain 5 percent by volume of the product. Also, as the propellant-to-product ratio is increased, the use life of the aerosol system is reduced due to the smaller quantity of the product which is present. This wll also make the aerosol product less attractive to the consumer.
In addition to the economic problems which are posed by aerosol systems that require a relatively large amount of propellant, there may also be compatibility problems between the propellant and product at high propellant-to-product volume ratios. For example, liquified hydrocarbon propellants are generally incompatibile with water base paints at the higher propellant-to-product volume ratios with the product forming a gel which cannot be ejected through a mechanical breakup nozzle in the form of a line spray. Halogenated hydrocarbon propellants, such as the products sold by E. I. DuPont de Nemours under the trade name "FREON" also have compatibility problems with various types of products. In order for a propellant to be used with a particular product, the propellant must be miscible, to some degree, with the product. However, halogenated hydrocarbon propellants are generally immiscible with water based products with the propellant and product forming separate layers and the propellant, thereby, not functioning to discharge the product as a fine spray through a spray nozzle.
In addition to compatibility problems, the use of a particular propellant in a given aerosol system may create still further problems. Hydrocarbon propellants are generally flammable in air at higher concentrations. Thus, when the driving force for the propellant system requires a high propellant-to-product volume ratio, a hydrocarbon propellant will generally be unsuitable for reasons of safety.
Also, halogenated hydrocarbon propellants are generally toxic and may form phosgene if contacted by flame. In addition, halogenated hydrocarbon propellants are expensive. Because of their toxicity, halogenated hydrocarbon propellants are generally unsuitable for use in an ingestible food product.
Due to the various problems of cost, safety, compatibility, flammability, toxicity, etc., as enumerated above, certain types of products are dispensed from containers in which the product is physically separated from a propellant that is used to discharge the product from the container. These types of containers, which may be termed "barrier packs", are exemplified by the barrier pack sold under the name "SEPRO" by Continental Can Company and that sold under the track name "POWER-FLOW" by American Can Company.
In a barrier pack, a flexible inner container for the product is progressively collapsed by the pressure generated by a propellant in the region between a rigid outer container and the flexible inner container. The product is, thereby, extruded through an opening leading from the collapsible inner container. If the product being dispensed from a barrier pack container is a foamable product, such as a shaving cream, the liquid product within the flexible inner container may also contain a propellant in admixture therewith. Then, when the product has been extruded from the inner container, the propellant within the product will undergo vaporization at ambient pressures to produce foam. Such an aerosol package is shown in the Lauwe patent, U.S. Pat. No. 3,788,521, dated Jan. 29, 1974.
The foregoing problems have sharply limited the usage of aerosol dispensing systems for various products and have generally precluded their use for products such as viscous water base products, ingestible food products, etc. Thus, it would be highly desirable to provide an aerosol dispensing system which would not be subject to the various drawbacks encountered with present aerosol systems and which could be used to dispense essentially any type of liquid or powdered product including viscous water base paints and ingestible food products.